Systems and methods for stirring electromagnetic fields and interrogating stationary rfid tags

ABSTRACT

RFID tags are used for many purpose including tracking RFID interrogators are used to retrieve information from tags. In many applications, RFID interrogators and RFID tags remain stationary during interrogation. Regions of low energy due to interference from either additional antenna or reflections from RFID tags and objects can impede or prohibit the reading of RFID tags residing in such regions. Stirring of the generated electromagnetic field is a method of moving around the regions of low energy, where tags can not be read, during the interrogation process. Mechanical stirring is accomplished by introducing a conductor into the electromagnetic field and moving it about in the field. Solid state stirring is accomplished by introducing a variable conductor into the field and varying the conductivity of the variable conductor. Mathematical stirring is accomplished by use of a plurality of antenna and controlling the phase difference between the antenna in a configuration known as phased antenna arrays.

RELATED APPLICATIONS INFORMATION

This application claims priority as a divisional under 35 U.S.C. 120 toU.S. Patent Application 11/766,752, filed Jun. 21, 2007, and entitled“Systems and Methods for Stirring Electromagnetic Fields andInterrogating Stationary RFID Tags,” which in turn claims priority under35 U.S.C. 119(e) to Provisional Patent Application Ser. No. 60/805,423,filed Jun. 21, 2006, and entitled “An RFID Smart Cabinet and aMulti-Document Read Write Station,” both of which are incorporatedherein by reference as if set forth in full.

BACKGROUND

1. Field of the Invention

The field of the invention relates generally to Radio FrequencyIdentification (RFID) systems and more particularly to systems andmethods for reading and writing information from multiple RFID enableddocuments.

2. Background of the Invention

FIG. 1 illustrates a basic RFID system 100, A basic RFID system 100comprises three main components: an antenna or coil 104; an interrogator102, and a transponder, or RF tag 106 which is often electronicallyprogrammed with unique information. Antenna 104 can be configured toemit radio signals 108 to activate tag 106 and read and write data fromthe activated tag 106. Antenna 104 is the conduit between tag 106 andinterrogator 102, which is typically configured to control dataacquisition and communication. Antennas 104 are available in a varietyof shapes and size. For example, in certain embodiments they can bebuilt into a door frame to receive tag data from persons or thingspassing through the door. In other embodiments, antennas 104 can, forexample, be mounted on an interstate toll booth to monitor trafficpassing by on a freeway. Further, depending on the embodiments, theelectromagnetic field, i.e., radio signal 108, produced by an antenna104 can be constantly present when, e.g., multiple tags 106 are expectedcontinually. If constant interrogation is not required, then radiosignal 108 can, for example, be activated by a sensor device.

Often antenna 104 is packaged with interrogator 102. A conventionalinterrogator 102 can emit radio signals 108 in ranges of anywhere fromone inch to 100 feet or more, depending upon the power output and theradio frequency used. When an RFID tag 106 passes through anelectromagnetic zone associated with radio signal 108, it detects radiosignal 108, which can comprise an activation signal. In someembodiments, interrogators can comprise multiple antenna, thoughtypically only one transmits at a time.

Additionally, interrogator 102 is often coupled through network 112 to acentral server 112. Central server 112 can be configured to execute anumber of applications including those which incorporate data from anRFID tags. For example, in a tracking system, interrogator 102 transmitsto the central server 112 the identity of tags which pass through itsinterrogation zone. This information can be correlated to objectsassociated with the tag in a database residing on the central server andhence the whereabouts of the object in question at that particular timecan be logged. In the example of a toll booth, tags that pass throughthe specific toll both are reported to central server 112 whichcorrelates the tag to a motorist who is then debited the cost of thetoll.

RFID tags 106 come in a wide variety of shapes and sizes. Animaltracking tags, for example, inserted beneath the skin, can be as smallas a pencil lead in diameter and one-half inch in length. Tags 106 canbe screw-shaped to identify trees or wooden items, or credit-card shapedfor use in access applications. Anti-theft hard plastic tags attached tomerchandise in stores can include RFID tags. In addition, heavy-dutyRFID tags can be used to track intermodal containers, heavy machinery,trucks, and/or railroad cars for maintenance and/or tracking purposes.

RFID tags 106 are categorized as either active or passive. Active RFIDtags 106 are powered by an internal battery and are typicallyread/write, i.e., tag data can be rewritten and/or modified. An activetag's memory size varies according to application requirements. Forexample, some systems operate with up to 1 MB of memory. In a typicalread/write RFID work-in-process system, a tag 106 might give a machine aset of instructions, and the machine would then report its performanceto tag 106. This encoded data would then become part of the taggedpart's history. The battery-supplied power of an active tag 106generally gives it a longer read range. The trade off is greater size,greater cost, and a limited operational life.

Passive RFID tags 106 operate without a separate external power sourceand obtain operating power generated from radio signal 108. Passive tags106 are consequently much lighter than active tags 106, less expensive,and offer a virtually unlimited operational lifetime. The trade off isthat they have shorter read ranges than active tags 106 and require ahigher-powered interrogator 102. Read-only tags 106 are typicallypassive and are programmed with a unique set of data, usually 32 to 128bits, that cannot be modified. Read-only tags 106 often operate as alicense plate into a database, in the same way as linear barcodesreference a database containing modifiable product-specific information.

RFID systems are also distinguishable by their frequency ranges.Low-frequency, e.g., 30 KHz to 500 KHz, systems 100 have short readingranges and lower system costs. They are commonly used in securityaccess, asset tracking, and animal identification applications.High-frequency, e.g., 850 MHz to 950 MHz and 2.4 GHz to 2.5 GHz 100systems offer long read ranges, e.g., greater than 90 feet, high readingspeeds, and are used for such applications as railroad car tracking andautomated toll collection, however, the higher performance ofhigh-frequency RFID systems 100 incurs higher system costs.

The significant advantage of all types of RFID systems 100 is thenoncontact, non-line-of-sight nature of the technology. Tags 106 can beread through a variety of substances such as snow, fog, ice, paint,crusted grime, and other visually and environmentally challengingconditions, where barcodes or other optically read technologies cannottypically be used. RFID tags 106 can also be read in challengingcircumstances at high speeds, often responding in less than 100milliseconds. RFID has become indispensable for a wide range ofautomated data collection and identification applications that would notbe possible otherwise

In other RFID systems the tags can remain relatively stationary. Forexample, in a warehouse tracking application, a forklift can be equippedwith an RFID interrogator, whose position and other motion informationcan be determined by reading RFID tags on the floor of the warehouse. Inthis application, the RFID tags are permanently affixed to the floor ofthe warehouse deployed in a known arrangement where the position of thetags are known ahead of time.

In a shipment tracking application, a handheld RFID interrogator can bepassed over a package to read an RFID tag. Typically, the operator canscan a package's RFID tag 106 by passing the interrogator 102 with theantenna 108 mounted on it or just the antenna 108 near the RFID tag 106.Though the package can be mobile as well, the interrogator 102 or itsantenna 108 are easily moved by the operator.

Still other RFID systems can have both stationary RFID tags 102interrogated by stationary interrogators 102. For example, in a medicalinventory system, medication in containers with RFID tags 106 affixed tothem are placed inside a cabinet or drawer. When the cabinet is closedor locked, an RFID interrogator 102 takes an inventory of the contentsof the cabinet. A central server 112 can compare the results of thisinventory to that of the inventory prior to the opening of the door,which yields a list of medication that was either added or removed fromthe cabinet. Such a system can be used to insure that the propermedication for a specific patient is removed for use.

An additional challenge in RFID interrogation arises when used in anenvironment where both the antenna and the RFID tags are stationary. Innormal transmission of electromagnetic energy, reflections from objects,RFID tags can cause destructive interference leading to regions in theelectromagnetic fields with either little or no energy. In addition, insystems where multiple antenna are used either by the same interrogatoror by a second interrogator. The electromagnetic fields generated bythese antennae can also destructively interfere leading to regions oflittle or no energy. In RFID applications where the tags are passedthrough a field, this phenomenon is not a problem since the tags aremoved through the field and only resides in one of these no energyregions for a very brief period of time. Likewise, when the interrogatoror interrogator's antenna is mobile, the field is then moved about thetags so these no energy regions are moved around the tags.

SUMMARY

RFID tags are used for many purpose including tracking RFIDinterrogators are used to retrieve information from tags. In manyapplications, RFID interrogators and RFID tags remain stationary duringinterrogation. Regions of low energy due to interference from eitheradditional antenna or reflections from RFID tags and objects can impedeor prohibit the reading of RFID tags residing in such regions. Stirringof the generated electromagnetic field is a method of moving around theregions of low energy, where tags can not be read, during theinterrogation process. Mechanical stirring is accomplished byintroducing a conductor into the electromagnetic field and moving itabout in the field. Solid state storing is accomplished by introducing avariable conductor into the field and varying the conductivity of thevariable conductor. Mathematical stirring is accomplished by use of aplurality of antenna and controlling the phase difference between theantenna in a configuration known as phased antenna arrays.

These and other features, aspects, and embodiments of the invention aredescribed below in the section entitled “Detailed Description.”

BRIEF DESCRIPTION OF THE DRAWINGS

Features, aspects, and embodiments of the inventions are described inconjunction with the attached drawings, in which:

FIG. 1 illustrates a basic RFID system.

FIG. 2 depicts a method of stirring an electromagnetic field by moving aconductor in the electromagnetic field.

FIG. 3 illustrates a system implementing the method of stirringdescribed in FIG. 2.

FIG. 4 illustrates another embodiment of a system implementing stirringwith an additional conductor.

FIG. 5 depicts a method of stirring an electromagnetic field using avariable conductor in the electromagnetic field.

FIG. 6 depicts a system implementing the solid state stirring of anelectromagnetic field during the interrogation of RFID tags.

FIG. 7 shows a system employed a phased antenna array.

DETAILED DESCRIPTION

It is well known in the electrical engineering arts that theintroduction of a conductor to an electric field alters the fieldprovided it is of a length greater than the wavelength of the field.Also, the change of position of the conductor also changes theelectromagnetic field. In particular, the location of low energy regionsresulting from destructive interference can be moved, spatially. Themethod of moving the low energy regions of an electromagnetic field bythe use of conductors is referred to as stirring.

FIG. 2 depicts a mechanical method of stirring an electromagnetic field.A conductor IS introduced into the electromagnetic interrogation fieldat step 210. As the interrogation zone is interrogated, the conductor isthen set in motion at step 212. In one embodiment, the conductor is onlyin motion while the RFID tags are being interrogated. In anotherembodiment, the conductor is in motion the entire time the system is inoperation. The motion of the conductor can be significant that is thedistance traversed is many times that of the wavelength. For simplicity,the motion can be a repeatable periodic pattern, such as rotation oroscillation.

FIG. 3 illustrates a system implementing the mechanical method ofstirring described in FIG. 2. In an embodiment, the RFID tags 302 areenclosed inside a structure 304. Interrogator 306 can reside outsidestructure 304, but antenna 308 can reside inside the structure toperform the interrogation. Conductor 310 can also reside insidestructure 304. In an embodiment, conductor 310 comprises and is coupledto shaft 312 that drives it in a rotational manner. The conductor 310can be, for example, a piece of metal. Shaft 312 is coupled to arotational mechanism 314, such as a motor, outside of the structure 304,which can be coupled to interrogator 306 so that the rotation occursonly when interrogation takes place or only when the interrogationsystem is active.

FIG. 4 illustrates another embodiment of a system implementing stirringwith an additional conductor 402 which can, for example, be a piece ofmetal. It is coupled to a shaft 404 which can be coupled to anoscillatory mechanism 406 which can be coupled to interrogator 306. Thedepiction of oscillatory mechanism 406 rather than a rotationalmechanism is presented to illustrate the variety of motion mechanismswhich can be employed to move the conductors within the electromagneticinterrogation field. The motion of conductor 402 can be arbitrary, but amore diverse stirring can be accomplished if the motion of conductor 310and conductor 402 are independent.

In another embodiment of the above described system, the conductors areplaced outside the portion of the structure housing the RFID taggedobjects. In such an embodiment, the antenna and conductor(s) can bebuilt into the door or walls of the structure or deployed external tothe structure. In additional embodiments, additional conductors withmotion independent of the other moving conductors in the system leads tomore diverse stirring of the low energy region of the electromagneticinterrogation field.

A solid state alternative to the described mechanical stirring of anelectromagnetic field is the use of a material of variable conductivity.A material that changes from an insulator to a conductor can alter anelectromagnetic field and in particular spatially move the low energyregion of an interrogation signal.

FIG. 5 depicts a solid state method of stirring an electromagneticfield. A variable conductor is introduced into the electromagneticinterrogation field at step 510. The variable conductor can be of alength greater than the wavelength of the electromagnetic field.Examples of variable conductors include a pin-diode (p-type intrinsic,n-type diode) which becomes a conductor when a voltage is applied to it,a photoconductor which becomes a conductor when exposed to light, or apiezoconductor which becomes a conductor when exposed to vibration ormechanical stress. As the interrogation zone is interrogated, theappropriate stimulus is applied to the variable conductor at step 512 tomake it a conductor, such as applying a voltage to a pin-diode or alight to a photoconductor. During the interrogation process, thestimulus is removed (step 514) and reapplied (step 512) in order to keepthe low energy region of the field in motion so that no RFID tag willreside within a low energy region throughout the interrogation process.In another embodiment of the method a second variable conductor isintroduced into the electromagnetic interrogation field at step 516 andas the interrogation zone is interrogated, the appropriate stimulus isapplied (step 518) and removed (step 520) throughout the interrogationprocess. The pattern of the stimuli applied to both variable conductorscan be independent, leading to a more diverse movement of the low energyregion of the interrogating electromagnetic field.

FIG. 6 depicts a system implementing the solid state stirring of anelectromagnetic field during the interrogation of RFID tags. RFID tags602 attached to tagged objects reside inside structure 604. Interrogator606 can reside external to structure 604. In another embodiment, it canreside internal to or as part of the wall of the structure. Antenna 608,which is coupled to interrogator 606 resides inside the structure, butin other embodiments can reside as part of the wall or just external tothe structure. Variable conductor 610 can be placed inside structure 604coupled to stimulus 612. In the case of a pin-diode, the stimulus can bea electrical potential wired to the pin-diode. In the case of aphotoconductor, the stimulus can be a optical fiber, light emittingdiode, or semiconductor laser mounted near the photoconductor. Thestimulus 612 can be coupled to the interrogator 606 or can beindependent of the interrogator 606.

In another embodiment of the system, another variable conductor 614 canbe placed inside structure 604, coupled to stimulus 616. Variableconductors 610 and 614 can be of similar or differing types, for exampletwo pin-diodes or a pin-diode and a photoconductor. Stimuli 612 and 616can be independent in their operation leading to a more diverse movementof the low energy region of the electromagnetic interrogation field.

In additional embodiments, additional variable conductors operatingindependently of the other variable conductors in the system leads tomore diverse stirring of the low energy region of the electromagneticinterrogation field.

Another method of shifting the position of low energy regions as well ashigh energy regions is by employing one or more additional antenna,where all antenna connected to the interrogator are coupled togetherwith a varying phase shift. This technique is referred to as phasedarraying of antennae. By varying the phase shift between the antennaethe high and low energy regions of the electromagnetic fields areshifted throughout the interrogation zone, thereby interrogating allRFID tags in the target region.

FIG. 7 shows a system employing a phased antenna array. Interrogator 702is coupled to antennae 704 and 706. Antennae 704 and 706 are linked byphase shifter 708, which can be controlled by interrogator 702. Antennae704 and 706 can be mounted just external to, inside the walls of, orinternal to the enclosing structure 710 as depicted in FIG. 7. Phaseshifter 708 varies the phase difference between antennae 704 and 706 sothat the field can scan the entire enclosing structure 710. Specificprescription for the shifting of phases to control the placement of thehigh and low energy regions is well known in, the art of radio frequencyengineering.

While certain embodiments of the inventions have been described above,it will be understood that the embodiments described are by way ofexample only. Accordingly, the inventions should not be limited based onthe described embodiments. Rather, the scope of the inventions describedherein should only be limited in light of the claims that follow whentaken in conjunction with the above description and accompanyingdrawings.

1. A method for interrogating RFID tags in a static interrogationenvironment comprising: providing an antenna for transmitting andreceiving RF signals to and from RFID tags; generating anelectromagnetic field of a predetermined frequency in the interrogationenvironment; and varying the electromagnetic field in the interrogationenvironment by introducing a conductor into the interrogationenvironment; and moving the conductor in the interrogation environment.2. A method for interrogating RFID tags in a static interrogationenvironment comprising: providing an antenna for transmitting andreceiving RF signals to and from RFID tags; generating anelectromagnetic field of a predetermined frequency in the interrogationenvironment; and varying the electromagnetic field in the interrogationenvironment,
 3. The method of claim 2, wherein the step of varying theelectromagnetic field in the interrogation environment comprises:introducing a conductor into the interrogation environment; and movingthe conductor in the interrogation environment.
 4. The method of claim2, wherein the step of moving the conductor in the interrogationenvironment is continuously moving the conductor in the interrogationenvironment.
 5. The method of claim 2, wherein the step of moving theconductor in the interrogation environment comprises rotating theconductor.
 6. The method of claim 2, wherein the conductor is a piece ofmetal having a length greater than the wavelength of the electromagneticfield.
 7. A method for interrogating RFID tags in a static interrogationenvironment comprising: providing a plurality of antennae fortransmitting and receiving RF signals to and from RFID tags; generatingan electromagnetic field of a predetermined frequency in theinterrogation environment; and maintaining and varying the phasedifference between the antenna as to move high energy field intensitiesacross the interrogation zone.
 8. A system for interrogating RFID tagsin a static interrogation environment comprising: an RFID tag; an RFIDinterrogator; a plurality of antenna configured to interrogate RFID tagsat a given frequency; and a phase shifter coupled to the plurality ofantenna and the RFID interrogator.
 9. The system of claim 8, furthercomprising an enclosure within which interrogation takes place.
 10. Thesystem of claim 8, further comprising an enclosure within whichinterrogation takes place a network coupled to the interrogator; and acentral server coupled to the network.